FIELD OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART
This invention relates to a method for drying plastic materials such as rubber and the like, wherein most or all of the water is removed by vaporization or by pressure squeezing, or both, but which has a multiplicity of pores or voids.
In the production of processed rubber and various synthetic materials, the product often contains water in varying proportions. Indeed, the starting material sometimes contains as much as 50% by weight of water or even more, and it is often desired to remove the water until the product contains less than about 1% by weight of residual moisture, or less, while nevertheless producing the product in the form of a multiplicity of readily workable particles.
For some time it has been the accepted commercial practice to extrude plastics and the like by utilizing one or more worms rotating in a barrel, thus causing a heating and working of the plastic which can be utilized for the purpose of removing solvent from the plastic. One such apparatus which has been highly successful for accomplishing this end is shown and claimed in the Fuller Reissue U.S. Pat. No. 23,948, granted to the assignee of this application on Feb. 15, 1955.
When efforts are made to utilize the apparatus that is shown in the aforementioned Fuller Reissue patent, to remove substantial quantities of water from material such as rubber particles or the like, the presence of excessive quantities of water seriously impedes the efficiency of the apparatus, such that when efforts are made to enlarge the scale of the feed end of the extrusion dryer shown in the Fuller Reissue patent, the mere enlargement of parts has been found to be an inadequate solution to the problem of removing excessive quantities of drainable water such as in the order of 50% by weight more or less.
In the past, rubber has been conventionally dried in extruders or other continuously operating machines, for example those disclosed in the patent to Skidmore and Fulton U.S. Pat. No. 3,323,222, which utilize pressure squeezing followed by drying on a fluid bed conveyor.
It has also been suggested, in the patent to Zies U.S. Pat. No. 3,222,797, to charge rubber and water into an extruder and to provide heat and pressure, trapping the water with the rubber and exploding rubber plus steam out through the die orifice in the extruder. This process provides an expansion of rubber but, in the process of expansion, the escaping steam undergoes expansion cooling, and some of the steam tends to condense, reintroducing water into the rubber.
It is also desirable in the art to produce a more workable rubber crumb which is dry and which can be introduced into a mold and there compressed in order to form a generally cube-shaped unit which is normally referred to as a bale. The art has experienced some difficulty in carrying out the bale forming operation, due to poor workability of the rubber crumb. Accordingly, one object of this invention is to provide a rubber crumb having improved workability.